The invention relates to a battery charger comprising a converter for converting an input voltage between an input terminal and a reference terminal into a terminal voltage of a battery between an output terminal and the reference terminal, comprising switching means for controlling the value of the terminal voltage to the value of a reference voltage, a control circuit for controlling the switching means, comparison means for making a comparison of the terminal voltage with the reference voltage, and indication means for providing an indication signal when the battery is substantially fully charged.
Such a battery charger is known from international patent application WO 94/14221. A battery charger is described therein which comprises a self-oscillating flyback converter with inter alia a transformer, a bipolar switching transistor, and a control circuit. The control circuit blocks the bipolar switching transistor when the terminal voltage of the battery has risen above a reference voltage. When the pulse width of the voltage at the base of the bipolar switching transistor comes below a certain limit, an indicator lamp indicates that the battery has been (substantially) fully charged.
There is a demand for simple and reliable battery chargers for charging batteries of hearing aids, provided with indication means for indicating that the battery is substantially fully charged.
To meet this demand, the battery charger mentioned in the opening paragraph is characterized in that the converter comprises a coil with a first and a second terminal; and in that the switching means comprise a first switch of which a control electrode is coupled to the control circuit for receiving a first control signal and a second switch of which a control electrode is coupled to the control circuit for receiving a second control signal; and in that the first terminal of the coil is coupled to the input terminal; and in that the first switch is connected between the second terminal of the coil and the reference terminal; and in that the second switch is connected between the second terminal of the coil and the output terminal; and in that the comparison means provide a start signal to the control circuit in response to said comparison for initiating an energy transfer from the input terminal to the battery; and in that the battery charger comprises frequency detection means coupled for receiving the start signal and coupled to the indication means for providing the indication signal when the frequency of the start signal has exceeded a certain limit.